Home security systems of various types have previously been developed. These systems use one or more sensors to detect one or more alarm conditions, such as an intruder, a fire, a drop in temperature due to a furnace failure, and so forth. These prior systems typically actuate an audible alarm, the purpose of which is to scare away any intruder, warn all persons present of the alarm condition, and to warn other persons in the immediate vicinity of the alarm condition. However, if there is no one in the building and if persons in the immediate vicinity do not hear and respond to the alarm, the system is rendered ineffective. For this reason, some prior systems have also been provided with a device which, when triggered, will automatically dial the police or a security service, but an intruder can defeat these systems by cutting the telephone lines to the building prior to entering the building. One approach to overcoming these problems, in particular with respect to making neighbors or other persons in the vicinity aware of an alarm condition, is to provide a system which can communicate with other systems in nearby buildings using radio waves, for example over citizens band channels, since citizens band transceivers are readily available at relatively low cost.
The Federal Communications Commission (FCC) has set aside 40 channels for citizens band radios, of which 6 can be used for coded signals such as radio control applications. Since the FCC made these channels available to the general public without examination requirements, there has been a great interest in using these channels for control and signaling purposes ranging from simple transmitter identification schemes to rather complex systems like those used for the remote control of model airplanes, boats, and cars. As a simple example, a person might like to avoid hearing the continual verbal chatter that is normally present on the typical citizens band channel by having a device connected to his receiver that would only permit an audio output when his receiver receives a unique signal transmitted specifically to him, for example by his neighbor or his spouse. The receiving station, although continually receiving radio signals generated by the transmitting station of interest and also all other citizens band stations within range, would thus product an audible output only when another transmitting station emitted the requisite unique signal. The person at the receiving end would then be called upon to listen to the extremely noisy conditions that prevail on the usual citizens band channel only when the person at the transmitting end was trying to reach him, rather than continuously.
Although such arrangements are easy to imagine, the situation is quite different in practice, because of a number of legal and physical restrictions imposed on citizens band systems.
First, unlimited Radio Frequency power is not available, because the Federal Communication Commission limits the RF power of a citizens band transceiver to 4 watts (except on one channel 23, which can be used with up to 25 watts). With simple antennas, this restricts the range of such systems to approximately five miles.
Second, the Federal Communications Commission forbids internal adjustment and modification of citizens band transceivers except by holders of the appropriate class of FCC license, and restricts rather severely the adjustments and modifications even those persons may make. In particular, modifications to increase power output and/or to change the modulation techniques are illegal. Consequently, a security and alarm system using citizens band transceivers would have to inject signals into an unmodified citizens band transceiver in the normal way, namely through the microphone input, and since citizens band transceivers are designed to accept voice signals in the audio range, the injected signals would have to be in that range of frequencies, for example from 300 Hz to 3000 Hz.
Third, the above-mentioned restrictions on internal modifications to citizens band devices would also limit the security and alarm system to observing the audio output of the receiver, which may not reproduce the waveform of a transmitted signal with great accuracy. In fact, only sinusoidal signals may be counted on to come through with a reasonably faithful degree of reproduction, due to the narrow audio bandwidth of the transceiver.
Fourth, the citizens band channels are continually filled with other interfering signals which are in themselves legal, since they originate from other licensed stations transmitting voice signals. Since these other transmitters are often mobile stations, the signals received are often very strong. Attempting to receive information from a station five miles away while a transmitter fifty feet away is transmitting is a challenging task, because the strong signals from the nearby transmitter will typically capture the automatic gain control loop of the receiver and thus suppress the signal from the remote transmitter.
These interfering signals can in a sense be referred to as "noise", and one might think that their effects can be readily overcome, because noise suppression and filtering techniques are highly developed and are widely used in the scientific, engineering, and radio communications field. However, the "noise" on the citizens band channels is quite different from the noise that communications technology can suppress, in that it is highly variable in intensity and spectral content with respect to time. That is, the citizens band "noise" is "nonstationary", whereas "stationary" noise has statistical properties such as amplitude distribution and power spectral density that do not change with time. Accordingly, it is far more accurate to think of the interfering signals as "jamming" signals which are highly variable in amplitude, frequency, and pattern of occurrence.
One approach to solving these problems is to start with a simple audio oscillator generating a precisely known frequency in the audio range, for example 1 KHz. This signal is in the passband of the typical citizens band transceiver, and will be transmitted as though it were a normal voice signal. At the receiving end, the 1 KHz signal will be received (if the interfering signals are sufficiently weak), and may be passed through a filter designed to pass only a narrow range of frequencies centered on 1 KHz. The output of this filter will be large only if a 1 KHz signal is being received, and could be taken as an indication that the transmitting station of interest was transmitting. A relay could then be closed, allowing the audio output of the receiver to reach an external loudspeaker or other form of audible alarm, thus enabling the person at the receiving end to hear what was being transmitted.
Many such simple systems have been designed and marketed. They do not work well, however, for the simple reason that normal speech patterns contain substantial amounts of energy in the frequency range surrounding 1 KHz, and this energy causes the narrow band filter to frequently respond to voice signals in exactly the same way that it would respond to the enabling signal from the transmitting station of interest.
An approach to improving the situation would be to pick a better frequency or use narrower filter bandwidths. Because of the restricted bandwidth of the CB transceiver, however, there aren't any frequencies significantly better, and as the receiving filter bandwidth is made narrower, it becomes technologically difficult to make sure that the transmitter and receiver are aligned to the same audio frequency.
Another approach is to use combinations of two or more frequencies transmitted simultaneously or sequentially in an attempt to make the triggering signal sufficiently different from voice signals so that the receiver may reliably tell the two apart. Many attempts have been made in this direction, but none have produced entirely satisfactory results. The problem of reducing the probability of a false alarm to sufficiently low levels while keeping the probability of detecting a true alarm sufficiently high for the system to fulfill its intended purpose is thus difficult. Utilizing relatively simple electronics, it is very hard to generate signals significantly different from those appearing as normal background chatter on the citizens band channels; female voices are particularly likely to trigger such devices with great regularity, due to their strong high frequency content.